A known exhaust purification system that reduces nitrogen oxide (hereinafter referred to as “NOx”) contained in exhaust, such as that described in patent document 1, includes a urea selective catalytic reduction (SCR) system using a urea water adding device and a selective reduction catalyst. In the urea SCR system, exhaust gas to which urea water has been added by the urea water adding device flows into the selective reduction catalyst, and the urea water is converted into ammonia through hydrolysis. In the selective reduction catalyst, NOx is reduced by the ammonia and converted into nitrogen and water.